1. Field of the Invention
This invention relates to methods for manufacturing masks for use in x-ray photolithography and more specifically to methods for depositing an x-ray opaque material (such as gold) on a mask in accordance with a predetermined pattern.
2. Description of the Prior Art
It is known in the art to use masks in photolithographic processes for manufacturing integrated circuits. Processing a silicon wafer (which typically possesses a 4", 5" or 6" diameter) to obtain a given number of integrated circuits from the wafer is expensive. Because the smaller the integrated circuit, the more integrated circuits can be obtained from a wafer of given size, smaller integrated circuits are less expensive to fabricate than larger integrated circuits. It is also known in the art that there is a limit to the size of circuit structures that one can obtain by using photolithographic techniques with visible light. Accordingly, it has been suggested that radiation in the x-ray portion of the spectrum can be used in photolithographic manufacturing processes, e.g., as described in U.S. Pat. No. 3,743,842, issued to Smith et al. to obtain smaller integrated circuits. In processes using x-ray photolithography, it is necessary to provide masks which selectively block x-ray radiation.
Prior art masks (e.g., mask 10 of FIG. 1) used in x-ray photolithography typically include a boron nitride membrane 12 stretched across a pyrex ring 14. As is known in the art, boron nitride is transparent to x-rays. Boron nitride membrane 12 is covered with a polyimide layer 15. Deposited on polyimide layer 15 is a patterned gold layer 16 which is opaque to x-rays. A similarly patterned layer of tantalum adhesive 18 is provided between the gold and the polyimide to bond the gold to the polyimide. In addition, an intermediate layer of silicon 20 is bonded to both pyrex ring 14 and boron nitride layer 12.
Masks such as the one illustrated in FIG. 1 are typically produced in a subtractive fashion, i.e., a to-be-patterned mask as illustrated in FIG. 2a is provided in which boron nitride membrane 12 is completely covered with polyimide layer 15, tantalum layer 18, gold layer 16, and a mask layer 22, typically tantalum. Mask layer 22 is patterned, exposing regions of gold layer 16. The exposed regions of gold layer 16 are then etched away, typically by a sputter etching process, thus exposing regions of tantalum layer 18. Thereafter, the exposed regions of tantalum layer 18 are etched away and the remaining portions of mask layer 22 are removed, leaving patterned gold 16 adhering to polyimide layer 15 by means of intermediate similarly patterned layer of tantalum 18 (see FIG. 1). This process is discussed in greater detail in "Advances in X-Ray Mask Technology" by the inventor of the present invention, published in Solid State Technology in 1984.
This process is known to have several problems. Specifically, during the etching of gold layer 16, gold redeposits around the exposed edges of the patterned gold, causing the wall of the gold layer typically to have an angle of 65.degree. to 70.degree. with respect to the horizontal (see FIG. 2b). As is known in the art, the intensity of the x-rays reaching the area under the mask is inversely related to the thickness of the gold between the x-ray source and the photoresist being exposed by the x-rays. Because the thickness of the gold varies under the sloped edges, the edge definition of the structures being photolithographically produced using such a mask is degraded.
It is known in the art to provide masks for use in x-ray photolithography using an additive process in order to avoid the problem of sloped gold walls. In such processes, a structure including a membrane 50 (FIG. 3a) is coated with an intermediate adhesive layer 52 (e.g., tantalum), a thin gold layer 54, and a photoresist layer 56. Photoresist layer 56 is then patterned, forming window region 58, thus exposing a portion of gold layer 54. The mask is then subjected to a gold deposition process in which a gold layer 60 is deposited in window region 58 (FIG. 3b). Thereafter, the wafer is subjected to an etching process in which photoresist layer 56 is removed, exposing portions of gold layer 54. The exposed portions of gold layer 54 and the portion of adhesive layer 52 lying thereunder are then removed, leaving the structure of FIG. 3c. Such a process is described in a paper entitled "DC Electroplating of Sub-micron Gold Patterns on X-ray Masks", by G. E. Georgiou et al.